The interaction between viral nucleic acids and proteins that lead to the formation of nucleoprotein structures is an area of biological investigation that has become increasingly important in the last several years. A powerful approach for understanding how these structures are assembled is through the study of model systems. The system that we have employed over the last three years, and which remains most useful at this time is the packaging of DNA into bacteriophage T4. In vivo studies with components blocked at several steps in head maturation have allowed us to determine some metabolic and structural requirements for head completion. We intend to extend these studies in order to specifically detail the role played by the host membrane, major and minor capsid polypeptides, DNA polymerase, phage-specific nucleases, and viral DNA in T4 nucleocapsid assembly. A combination of biophysical, biochemical, and genetic techniques will be implemented in this research program. Concurrently, the morphopoiesis of Friend leukemia virus nucleocapsids will be examined in mouse embryo cells by immuno-electron microscopy using antibody against specific viral core substructures (i.e., core shells and enclosed ribonucleo-protein (RNP) strands). Emphasis will also be placed on an analysis of isolated plasma membranes from infected cells on a biochemical, immunological, and electron microscopic level, to ascertain the relationship between core shell and RNP polypeptides, with host cell membrane components. In addition studies will be performed to compare cores from early (less than 3 hours) or late (24 hours) viral harvests, on a morphological, biochemical, and DNA polymerase activity level. This latter project should aid in clarifying the relationship between immature and mature "C" type particles.